Method for enhancing the measuring accuracy in an antenna array

ABSTRACT

The invention refers to a method and a system for enhancing the measuring accuracy in an antenna array ( 1 ), where the method comprises the steps of; a) receiving analog signals on all antenna elements ( 2 ) at a first time t 1 ; producing first values for a first radiation diagram from the values in the signals from the first time (t 1 ), and; finding the maximum point ( 8 ) for the first values, b)—reducing the signal from one interadjacent antenna element ( 2 ) at a second time (t 2 ); receiving analog signals on all antenna elements ( 2 ) except from the one switched off or reduced antenna element, and; producing second values for a second radiation diagram from the values in the signals from the second time (t 2 ); c) rejecting all values outside a first range ( 9 ) calculated from the first values, and; finding the maximum point ( 8 ) for the second values.

This application is the US national phase of international applicationPCT/SE2002/001550 filed 30 Aug. 2002 which designated the U.S., theentire content of which is hereby incorporated by reference.

TECHNICAL FIELD

The invention refers to enhancing the measuring accuracy in an antennaarray comprising a number of antenna elements. A method comprises theacts of;

-   -   receiving analog signals on a number of m antenna array        elements, and;    -   producing a radiation diagram for the array from the values in        the signals.

The technololgy also refers to an antenna array system comprising

BACKGROUND ART

In the field of antenna array systems it is well known to use antennaelements in the antenna array to shape a beam sent out from the antennaarray. It is also known to let all the antenna elements in the antennaarray receive signals. When the antenna array receives the signals it ispossible to use one or several of the antenna elements, or evensub-array systems comprising a number of antenna elements. The antennaarray can be used, for example, in a radar system or a sonar system andis intended to be used in trying to estimate the direction-of-arrival ofa target.

When using the antenna array applications there is a wish to obtain highresolution and accurate estimation of the direction-of-arrival of thetarget. In order to gain the best performance possible it is commonknowledge that there has to be a trade off between the standarddeviation σ (or variance σ²) of the angle for detecting the target andthe SNR (Signal to Noice Ratio). The higher the SNR the lower thestandard deviation. The standard deviation is coupled to the probabilityof finding the target. The higher the standard deviation the lower theprobability. The so-called “Cramér-Rao Lower Boundary (CRB)”, definesthe theoretically best ratio between the SNR and the standard deviationσ for Additive White Gaussian noise (AWGN) signals. It is the desire ofevery antenna user to have a system that performs as close as possibleto the CRB. This is due to the fact that for a given SNR the lower thestandard deviation the closer to the CRB, i.e. the better the accuracyof direction-of-arrival estimation of a target.

However, the SNR is also coupled to the performance of the antennasystem and the size of the targets. The performance refers to theprobability of estimating the direction of arrival of a target. Theaccuracy depends on the width of the top of the main lobe, if the targetis represented by the main lobe. The higher the SNR the more narrow thetop of the main lobe. It is the tapering of the main lobe and thepointyness of the main lobe that tells where the maximum of the mainlobe can be found in a radiation diagram. The more pointed the lobe thebetter the measuring accuracy when finding the main lobe maximum, i.e.the better measuring accuracy when estimating the direction of arrivalof a target.

The lower limit for the SNR, i.e. the lowest performance possible forthe antenna system, occurs where the noise in the signal drowns thesignal from the target. This becomes clear if one follows the CRB whendiminishing the SNR. The standard deviation increases with decreasingSNR i.e. it becomes more difficult to correctly estimate the directionof arrival of the target the lower the SNR. A strong signal compared toa low noise gives a high SNR and a low uncertainty of the estimation ofthe direction-of-arrival of the target, and vice versa for a low SNR.

It is a desirable feature for an antenna system to have the ability todetect and estimate the direction of arrival of the target with areasonable probability (reasonably low standard deviation). An optimumis thus sought for the trade off between low standard deviation and lowSNR.

As has been stated above, one way to obtain an antenna system with gooddirection finding ability is to narrow the main lobe. This can becarried out by separating the antenna elements in the antenna array. Themore separated the elements are the more narrow the main lobe becomesand thus the better direction finding ability of the system.

However, the separation of the antenna elements give rise to gratinglobes due to the so-called Spatial Aliasing Phenomena. The problem withgrating lobes occurs when the antenna elements are separated by morethan half a wavelength λ, i.e. at the Nyqvist frequency. The gratinglobes are mathematical products that will appear in an antenna diagramshowing a radiation diagram of the gain G (θ) versus the azimuth angleθ. The integral over the radiation diagram is constant independent onthe size of the main lobe and the size and number of the grating lobes,i.e. the more grating lobes the lesser and narrower the main lobe.

The grating lobes will appear on each side of the main lobe and withdecreased amplitude the further away from the main lobe they are found.The two grating lobes closest to the main lobe have the highestamplitude. The grating lobes are thus dependent of the angle and can beinterpreted as signals from the main lobe seen from the side angle θ.

The grating lobes cause problems when trying to detect the direction ofarrival of a target. The target will randomly skip between the gratinglobes for low SNR and will therefore create random errors regarding thedetection probability of the target. Thus, the grating lobes generate ahigh standard deviation.

As has been described above, the more separated the antennas are, thelarger the antenna becomes and the narrower the main lobe. The narrowerthe main lobe the better the direction detection probability, i.e. thebetter the estimation of direction-of-arrival. However, the moreseparated the antennas are, the more and the higher the grating lobeswill appear in the radiation diagram of the antenna array.

To sum up the above, when the antenna elements are separated far enoughto give a narrow enough main lobe to get a good estimation ofdirection-of-arrival, the grating lobes will cause an uncertaintybecause the target skips between the main lobe and the grating lobes.

It is an object of the technology to diminish random errors regardingthe resolving probability of the target when trying to narrow the mainlobe, in order to get better estimation of the direction-of-arrival of atarget. It is thus an object of the technology to eliminate the gratinglobe problem when trying to “zoom in” on a target, i.e. to achieve abetter measuring accuracy.

BRIEF SUMMARY

The technology intends to meet the above objectives with a method forenhancing the measuring accuracy in an antenna array comprising a numberof antenna elements.

The method comprises the acts of;

-   -   receiving analog signals with the antenna array elements, and;    -   producing values for a radiation diagram from the values in the        signals, where the radiation diagram displays a main lobe and        grating lobes when present.

The analog signals are advantageously converted to digital signals bysampling and the radiation diagram is advantageously produced from thevalues in the digital signals.

The radiation diagram presents the values from the received signal asthe gain G (θ) versus the azimuth angle θ. The gain G (θ) for thedifferent angles θ are produced by using the antenna elements in pairsfor calculating different values in the received signal at differentangles. The antenna array is advantageously operational where the angle(θ) is varied between −π/2−π/2. The values for the different angles aremeasured by using the different antenna elements in pairs. In order tobe able to measure the effect for different angles for the incomingsignal, the antenna array uses at least two antenna elements that areomnidirectional.

The technology is characterized in that the method comprises the actsof;

a)—receiving analog signals on all antenna elements at a first time t₁;

-   -   producing first values for a first radiation diagram from the        values in the signals from the first time t₁, and;    -   finding the maximum point for the first values:

b)—switching off or reducing the signal from one interadjacent antennaelement at a second time t₂;

-   -   receiving analog signals on all antenna elements except from the        one switched off or reduced antenna element, and;    -   producing second values for a second radiation diagram from the        values in the signals from the second time t₂;

c)—using the first values to calculate a first range referring to thesecond radiation diagram, outside which first range grating lobes willappear in the second radiation diagram;

-   -   rejecting all values outside the first range, thereby excluding        the ambiguities in the second radiation diagram due to grating        lobes, and;    -   finding the maximum point for the second values.

The first part of act a) is to receive analog signals on all antennaelements at a first time t₁. The antenna elements are advantageouslyplaced in the antenna array with their relative distance equal to orgreater than the wavelength divided by two. The wavelength refers to thefrequency used in the antenna array system. The relative distance of theantenna elements fulfills the Nyqvist criteria which is which is whythere will be no grating lobes appearing in the radiation diagram whenusing a full array.

The expression “finding or calculating the maximum point for thevalues”, refers to finding the maximum point for the main lobe, i.e.finding or calculating the maximum value for the gain in the radiationdiagram. Hence, the method intends to find or calculate at what angleθ_(max) the maximum point for the main lobe will appear in the radiationdiagram.

The maximum value is sought for the values that are represented in theradiation diagram. The values are preferably saved in a memory as avector comprising the gain G (θ) versus the azimuth angle θ. There are anumerous techniques known for calculating the maximum value. One way tocalculate the maximum value is to search the vector for the maximumvalue for the gain and to find out at what angle the maximum value ispresent. Another way to find the maximum value is to make a graphicalcalculation, for example trying to find the solution to the equationwhere the derivative is zero.

Below the technology will be described as finding values in theradiation diagrams. However, the technology may instead use thetechnique of finding the values in the vectors generating the radiationdiagrams. The two techniques shall be seen as essentially equivalent toeach other.

According to act b), the antenna elements may be switched off or reducedby using antenna elements that may be manipulated, for exampleelectronically.

By switching off interadjacent antenna elements at certain times, thetime domain is used in order to achieve a better measuring accuracy. Asmentioned above, the more separated the antenna elements the narrowerthe main lobe. However, the more separated the antenna elements the moregrating lobes will appear.

By using the result from the first time t₁, it is possible to calculatea first range outside which range the grating lobes will appear in thesecond radiation diagram. Such calculation is known in prior art and maybe carried out by using the relative distance between two antennaelements and the frequency used in the antenna array system. Thiscalculation is also functional for subgroups of antenna element.

The range is calculated dependent on the antenna array configuration atthe time. Below an example will be given in order to clarify the methodof calculating the range.

W denotes the antenna array configuration at a certain time and thenumbers denotes different subgroups of the antenna array configurationat the time. X denotes active antenna elements and a hyphen denotes aswitched off or reduced antenna element.

W: xxx---xxx

This may be described in vector form as:W(k)=[111000111] (1×9 vector)  (1)

The antenna array configuration may be divided into:

W1: x-----x

Vector form:W1(k)=[1000001] (1×7 vector)  (2)

and:

W2: x x x

Vector form:W2(k)=[111] (1×3 vector)  (3)W(k)=W1(k)*W2(k) (1×9 vector)  (4)where * denotes convolution.

From (2) it is possible to calculate the range by using a Fouriertransform on (2). After that, the transform is set to zero and thecomplex zero that has the least angle is identified. The complex zerowill appear on the unity circle as a pair of complex conjugates. Therange may now be found as the distance between the complex zeroes alongthe unity circle. In order to get the distance in radians, asubstitution of variables may be done according to:θ=arcsin(u/dk)  (5)where u=the variable used in the Fourier transform and where d=thedistance between the antenna elements and where k=the wave number.

If the change in the array configuration is known in advance, the rangemay be calculated in advance and may then be applied in the differentradiation diagrams. The calculations may advantageously be done by acomputer or the like.

The first range may be used for rejecting all parts of the secondradiation diagram outside the first range, thereby excluding theambiguities in the second radiation diagram due to grating lobes.

The first range determines where the maximum point of the main lobe canbe found and where no grating lobes will appear. As mentioned before,the grating lobes may cause ambiguities since one of the grating lobesmay appear to be the maximum point for the radiation diagram. If one ofthe grating lobes is used as the maximum point for the radiationdiagram, the estimation of the direction of arrival of the target willbe wrong. By using the first range, all grating lobes may be rejectedand the maximum point for the main lobe can be calculated without anyambiguities stemming from the grating lobes.

After rejecting the grating lobes, the second radiation diagram may beused for calculating at which angle θ the maximum point for the mainlobe appears in the second radiation diagram, with a narrower main lobethan in the first radiation diagram.

Act b) and act c may be repeated in time until the antenna arraycomprises only the two outermost antenna elements. This antennaconfiguration generates the narrowest main lobe with the lowestamplitude and the highest grating lobes. Thus, this configurationproduces a radiation diagram where it may be difficult to find themaximum point for the main lobe. The grating lobes may actually be ashigh as the main lobe for some antenna configurations.

However, a new range and a new maximum have been calculated for everytime when the antenna array configuration has changed. The grating lobesmay be disregarded at each time and the maximum point for the main lobemay thus be calculated for the antenna configuration with only theoutermost antenna elements. This enhances the measuring accuracy for theantenna array configuration and enables a better estimation of directionof arrival of a target.

The radiation diagrams comprise the values of the gain G(θ) versus theazimuth angle θ. Since the number of antenna elements at the second timet₂ is less than the maximum number of antenna elements, grating lobeswill appear in the second radiation diagram. Consequently, since theantenna configuration at a third time t₃ has even lesser antennaelements, more grating lobes will appear in a third radiation diagram.In the second and third radiation diagram the grating lobes appear atdifferent angles dependent on which and how many antenna elements thathas been switched of or reduced. In the second radiation diagram thefirst grating lobes appear on each side of the main lobe at certainangles, e.g. θ_(t2)=X_(t2) radians (or degrees) and θ_(t2)=−X_(t2)radians (degrees). In the third radiation diagram the first gratinglobes appear on each side of the main lobe at certain angles, e.g.θ_(t3)=X_(t3) radians (or degrees) and θ_(t3)=−X_(t3) radians (degrees).According to the technology, X_(t2) is greater than X_(t3) and −X_(t2)is lesser than X_(t3), i.e. the first grating lobes in the thirdradiation diagram appear closer to the main lobe than the first gratinglobes in the second radiation diagram. The third radiation diagrampresents a second range outside which the grating lobes will appear.According to the above the second range must be narrower than the firstrange.

However, the main lobe in the second and third radiation diagramsappears within every corresponding range if the SNR is high enough.Furthermore, the main lobe is narrower in the third radiation diagramthan in the second radiation diagram and the second range is narrowerthan the first range, which is why the third radiation diagram shows abetter measuring accuracy than the second radiation diagram afterrejecting values outside each of the ranges.

In another example embodiment, a number of radiation diagrams will beproduced for every antenna array configuration. The number of radiationdiagrams may be used for calculating a mean value for the antenna arrayconfiguration, i.e. a mean value for the gain at different angles. Amean value may be calculated by any known methods e.g. Bartlett.Furthermore, when calculating a number of radiation diagrams for eachtime, it is possible to reject all radiation diagrams where the maximumis found/calculated outside the range. This increases the measuringaccuracy since the desired maximum value for the main lobe may be foundin at least one radiation diagram.

The technology may thus be used by dynamically altering the antennaarray such that interadjacent antenna elements are switched off orreduced until only the outermost antenna elements remain.

The benefits of the technology will become apparent when describing theexample embodiments below.

The amplitudes of the main lobes G_(t1)(θ_(main)), G_(t2)(θ_(main)) donot have to be global maximums in the radiation diagram, but may belocal maximums. If the main lobe is not a global maximum, there will bean error in the resolution since the maximum point will be found outsidethe range and the grating lobes will thus generate a drastic increase inthe standard deviation.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with reference to the drawingsbelow.

FIG. 1 shows an antenna array according to one example embodiment with anumber of t₁-t₄ configurations in time.

FIG. 2 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ at the first time t₁ for the antennaarray in FIG. 1.

FIG. 3 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ at the second time t₂ for the antennaarray in FIG. 1.

FIG. 4 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ at the third time t₃ for the antennaarray in FIG. 1.

FIG. 5 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ at the fourth time t₄ for the antennaarray in FIG. 1.

FIG. 6 shows the radiation diagram in FIG. 3 overlapping the radiationdiagram in FIG. 2.

FIG. 7 shows the radiation diagram in FIG. 4 overlapping the radiationdiagram in FIG. 3.

FIG. 8 shows the radiation diagram in FIG. 5 overlapping the radiationdiagram in FIG. 4.

FIG. 9 schematically shows a graph depicting Cramér-Rao Lower Boundary(CRB), and the standard deviation σ versus the Signal to Noice Ratio(SNR) when using reduced antenna-array.

FIG. 10 schematically shows a two-dimensional antenna array withdifferent configurations at different times.

FIG. 11 diagrammatically shows a block diagram over the method accordingto one example embodiment.

FIG. 12 diagrammatically shows a block diagram over the method accordingto the invention according to one embodiment.

DETAILED DESCRIPTION

The description of the different embodiments below shall not be seen aslimiting for the invention, but merely as examples within the scope ofthe claims.

An antenna array 1 uses antenna elements 2 for receiving analog signals.The antenna array 1 is connected to an analog to digital converter thatsamples the analog signals and creates a digital signal. The digitalsignal comprises values from the analog signal at certain points intime. The values may be represented as the gain G(θ) versus the angle θ.The values may be presented in a radiation diagram in order toillustrate the strength of the analog signal for different angles atcertain points in time.

FIG. 1 shows an antenna array 1 according to one example embodimentcomprising five antenna elements 2, with a number of configurations intime. The antenna elements 2 are depicted with a large X. At the firsttime t₁ the antenna array 1 uses all of the antenna elements 2. At thesecond time t₂ the antenna array 1 uses all but one of the antennaelements 2. At the third time t₃ the antenna array 1 uses all but two ofthe antenna elements 2. At the fourth time t₄ the antenna array 1 usesall but three of the antenna elements 2. As can be seen in FIG. 1, theconfiguration of the antenna array 1 at the fourth time t₄ uses only thetwo most widely separated antenna elements 2.

“Not using” an antenna element or “removing” one antenna element; meansthat the signals from the antenna array 1 are reduced or blocked. Thisis advantageously done before the sampling of the signals, but may becarried out after the sampling. However, if the antenna elements 2 areto be reduced or blocked after the sampling, the system should requireone sampling device per antenna element. In FIG. 1 the reduced antennaelements 2 are depicted with a small x and denoted with 2′.

In FIG. 1 the distance between the antenna elements is denoted 3, 4, 5.The distance between two antenna elements with a reduced antenna elementin-between is denoted 4. The distance between two outermost antennaelements is denoted 5.

The use of different antenna elements 2 according to above increases thephase centre between subgroups of antenna elements 2. At the fourth timet₄, the distance 5 between the two outermost antenna elements 2 givesthe maximum increase in phase centre for the antenna array 1 accordingto the embodiment. As has been described before, the increase indistance between the antenna elements 2 narrows the main lobe 6 (seeFIG. 2) but decreases the amplitude. One drawback is the generation ofgrating lobes 7 (see FIG. 2) which may cause an increase in the standarddeviation. However, for every increase in distance between the antennaelements 2 according to above, the closer to the main lobe 6 the gratinglobes 7 will appear.

FIG. 2 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ for a full antenna array 1 correspondingto the first time t₁. If the distance 3 between the antenna elements 2is equal to or less than half the wavelength, no grating lobes 7 (seeFIG. 3) will appear. In this embodiment the distance 3 between theantenna elements 2 is equal to or less than half the wavelength. In FIG.2 this becomes apparent since only a main lobe 6 appears in theradiation diagram. In this embodiment, there are no errors in theestimation of the direction-of-arrival since no grating lobes 7 appear.The maximum point 8 for the radiation diagram coincides with the apex ofthe main lobe 6. However, the width of the top of the main lobe 6 givesrise to uncertainty regarding the estimation of thedirection-of-arrival.

As can be seen in FIG. 2, the antenna array 1 is advantageouslyoperational where the angle (θ) is varied between −π/2−π/2.

In FIG. 2 the main lobe 6 appears within the range of −π/2−π/2.

FIG. 3 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ for a reduced antenna array 1corresponding to the second time t₂. Here the distance between thecentral antenna elements 2 is more than half the wavelength λ which iswhy first grating lobes 7 appear in the radiation diagram. In FIG. 3this becomes apparent since both a main lobe 6 and two grating lobes 7appear in the radiation diagram. In this embodiment, the first gratinglobes 7 give rise to an uncertainty regarding the estimation of thedirection-of-arrival.

The maximum point 8 for the radiation diagram coincides with the apex ofthe main lobe, but the main lobe 6 has a lesser amplitude than at thefirst time t₁. However, the width of the top of the main lobe 6 isnarrower than at the first time t₁ but still gives rise to uncertainty.

In FIG. 3, the first grating lobes 7 appear outside a first range 9,essentially symmetrically around the main lobe 6. The first range 9extends between the minimum points 12 between the main lobe 6 and thefirst grating lobes 7.

FIG. 4 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ for a reduced antenna array 1corresponding to the third time t₃. Here the distance 6 between thecentral antenna elements 2 is even greater than at the second time t₂which is why even more grating lobes 7 appear in the radiation diagram.In FIG. 4 this becomes apparent since both a main lobe 6 and severalgrating lobes 7 appear in the radiation diagram. The first grating lobes7 at the third time t₃ appear closer to the main lobe 6 than at thesecond time t₂ and also have greater amplitude. In this embodiment, thegrating lobes 7 give rise to even greater uncertainty regarding theestimation of the direction-of-arrival, since they are higher and themain lobe 6 smaller.

The maximum point 8 for the radiation diagram coincides with the apex ofthe main lobe, but the main lobe 6 has a lesser amplitude than at thesecond time t₂, which is why the ratio between the main lobe 6 and thefirst grating lobes 7 has diminished compared to the ratio at the secondtime. However, the width of the top of the main lobe 6 is narrower thanat the first time t₁ but still gives rise to uncertainty.

In FIG. 4, the grating lobes 7 appear outside a second range 10,essentially symmetrically around the main lobe 6. The second range 10extends between the minimum points 12 between the main lobe 6 and thefirst grating lobes 7.

FIG. 5 schematically shows a radiation diagram, with the gain in thesignal G(θ) versus the angle θ for a reduced antenna array 1corresponding to the fourth time t₄. In this embodiment the antennaarray 1 configuration only comprises the two outermost lying antennaelements 2. This may be compared to a spatial two-element comb filter.Here the distance 6 between the central antenna elements 2 is thus evengreater than at the third time t₃ which is why even more grating lobes 7appear in the radiation diagram. In FIG. 5 this becomes apparent sinceboth a main lobe 6 and several grating lobes 7 appear in the radiationdiagram. The first grating lobes 7 at the fourth time t₄ appear closerto the main lobe 6 than at the second time t₂ and also have greateramplitude. Actually, all grating lobes 7 in FIG. 5 theoretically havethe same amplitude. (compare to the previously known spatial two-elementcomb filter) In this embodiment, the grating lobes 7 give rise to evengreater uncertainty regarding the estimation of thedirection-of-arrival.

The maximum point 8 for the radiation diagram coincides with the apex ofthe main lobe, but the main lobe 6 has a lesser amplitude than at thethird time t₃, which is why the ratio between the main lobe 6 and thefirst grating lobes 7 has diminished compared to the ratio at the thirdtime. In FIG. 5 the amplitudes of the first grating lobes 7 areessentially as high as for the main lobe 6. This configuration of theantenna array 1 gives rise to a large uncertainty for the estimation ofthe direction-of-arrival of a target, since it is not obvious that thetarget will appear in the main lobe, but may appear in one of theclosest grating lobes 7.

However, the width of the top of the main lobe 6 at the fourth time t₄is the most narrow compared to all of the other antenna configurations.If it is certain that the target appears in the main lobe, the narrowtop of the main lobe 6 gives a good estimation of direction-of-arrival.One problem thus to be solved is the uncertainty which is dependent onthe grating lobes 7.

In FIG. 5, the grating lobes 7 appear outside a third range 11,essentially symmetrically around the main lobe 6. The third range 11extends between the minimum points 12 between the main lobe 6 and thefirst grating lobes 7.

One important aspect, however, is that the centre line of the main lobe6 for FIGS. 2-5 is mainly stationary (if the surrounding is stationaryfor all the antenna array 1 configurations in time), i.e. has not moved,for all the times t₁-t₄, but the first grating lobes 7 have moved closerto the main lobe 6 the more separated the antenna elements 2 are. Thecentre line for the main lobe 6 at different times may change somewhatdependent on the magnitude of noise or other phenomena that may disturbthe signal. However, the main lobe 6 will always appear within thecalculated range which is why the maximum point 8 for the main lobe 6 ateach time t_(n) is easy to calculate.

FIG. 6 shows the radiation diagram in FIG. 2 overlapping the radiationdiagram in FIG. 3. As can be seen in FIG. 6, the centre line of the mainlobes 6 has not moved. However, the grating lobes 7 from the radiationdiagram in FIG. 3 (i.e. at the second time t₂) have moved closer to thecentre of the main lobes 6 than the grating lobes 7 from the radiationdiagram in FIG. 2 (i.e. at the first time t₁). Furthermore, it becomesobvious from FIG. 6 that the main lobe 6 at the second time t₂ is morenarrow but with less amplitude than the main lobe 6 at the first time.FIG. 6 also shows the second range 10 being smaller than the first range9.

FIG. 7 shows the radiation diagram in FIG. 3 overlapping the radiationdiagram in FIG. 4. As can be seen in FIG. 7, the centre line of the mainlobes 6 has not moved. However, the grating lobes 7 from the radiationdiagram in FIG. 4 (i.e. at the third time t₃) have moved closer to thecentre of the main lobes 6 than the grating lobes 7 from the radiationdiagram in FIG. 3 (i.e. at the second time t₂). Furthermore, it becomesobvious from FIG. 7 that the main lobe 6 at the third time t₃ is morenarrow but with less amplitude than the main lobe 6 at the second timet₂. FIG. 7 also shows the third range 11 being smaller than the secondrange 10.

FIG. 8 shows the radiation diagram in FIG. 4 overlapping the radiationdiagram in FIG. 5. As can be seen in FIG. 8, the centre line of the mainlobes 6 has not moved. However, the grating lobes 7 from the radiationdiagram in FIG. 5 (i.e. at the fourth time t₃) have moved closer to thecentre of the main lobes 6 than the grating lobes 7 from the radiationdiagram in FIG. 3 (i.e. at the third time t₃). Furthermore, it becomesobvious from FIG. 8 that the main lobe 6 at the fourth time t₄ is morenarrow but with less amplitude than the main lobe 6 at the third timet₃. FIG. 8 also shows the fourth range being smaller than the thirdrange 11.

The grating lobes 7 will appear closer to the main lobe 6 for each timethat an antenna element 2 is removed and the main lobe 6 will becomenarrower. In figs. 6-8 the advantage of the technology is apparent,where the rejection of the grating lobes 7 for the different timesgenerate a possibility to gain a narrow main lobe 6 that enables a goodmeasuring accuracy of a target.

FIG. 9 schematically shows a graph depicting the Cramér-Rao LowerBoundary (CRB) and the standard deviation σ versus the Signal to NoiseRatio (SNR). The standard deviation is a deviation of the angle. FIG. 9also shows a first point P₁ at a certain distance from CRB and a secondpoint P₂ at a second location. The first point P₁ has a higher SNR andconsequently a lower standard deviation σ than the second point P₂.However, a lower SNR allows more noise in the signal, which is why theantenna array 1 may be used for detecting a target at a further distance(or a smaller target) than with a system at the first point P₁, if theincrease in the standard deviation can be accepted.

The first point and the second point P₁, P₂ symbolise an antenna array 1system with decreased SNR from the first point P₁ to the second point P₂along the CRB. At P₂ there is a threshold to a third point P₃. Thethreshold is a result of grating lobes 7 appearing at lower SNR. Thegrating lobes 7 give rise to an uncertainty of the angular estimation tothe target.

FIG. 9 shows that a SNR higher than at point two, P₂, gives a goodprobability of estimation of direction-of-arrival of a target at themain lobe 6. However this is only true if the maximum point of theradiation diagram coincides with the maximum point 8 of the main lobe 6.With a lower SNR than at the second point P₂, it is not certain that themaximum point of the radiation diagram is where the maximum point 8 forthe main lobe 6 is, but the maximum point might be deemed by the systemto be at one of the closest grating lobes 7. If this is the case, theestimation of direction-of-arrival of a target will be carried out atone of the grating lobes 7.

If one of the grating lobes 7 has been deemed by the system to be themaximum point and if the amplitude of the grating lobes 7 is lesser thanthe amplitude of the main lobe, the distance from the top of the mainlobe 6 to the top of the grating lobes 7 may be greater than the widthof the top of the main lobe 6. This gives rise to a considerableincrease in the uncertainty for the estimation of direction-of-arrivalof a target and thus an increase of the standard deviation.

When reducing the array according to the technology the width of themain lobe 6 will decrease, and the threshold will move closer to the CRBwith a demand for a higher SNR, than for the previous configuration ofthe array. This is depicted in FIG. 9 where the second point P₂ hasmoved to the fourth point P₄.

As pointed out before, the decrease of the main lobe 6 width decreasesthe standard deviation σ, but puts a demand for a higher SNR. This hasbeen pointed out in FIG. 9 as moving the first, second and the thirdpoints P₁, P₂ and P₃ to the corresponding fourth, fifth and sixth pointsP₄, P₅ and P₆.

As pointed out before, one problem with separating the antenna elements2 is that at one point, grating lobes 7 will appear in the radiationdiagram due to the separation. The grating lobes 7 will generate adecrease of probability of estimation of direction-of-arrival of thetarget. The target will randomly appear in the grating lobes 7 as adefect estimation of the direction-of-arrival. The sudden appearance ofgrating lobes 7, and thus the decrease of probability, is manifested inFIG. 9 as a jump from the second point P₂ to a third point P₃. The thirdpoint P₃ has a higher degree of standard deviation σ than the secondpoint P₂ for essentially the same SNR.

According to the technology, the rejection of the grating lobes 7outside the ranges for the different radiation diagrams, removes thejump from the second point P₂ to a third point P₃ as well as the jumpfrom the fifth point P₅ to a sixth point P₆. This is due to the factthat there will be no errors when calculating the maximum point 8 forthe main lobe 6 since there are no maximum points from grating lobes 7that can give rise to an error. In fig 9, this is depicted as moving thethird point P₃ to the third point prime P₃′ and as moving the sixthpoint P₆ to the sixth point prime P₆′.

FIG. 10 shows a two-dimensional antenna array 1 system according to oneexample embodiment, with a number of configurations in time t_(l),t_(m), t_(n). The antenna array 1 system comprises five rows along anY-axis. Each row comprises ten antenna elements 2 along a Z-axis. Theantenna elements 2 are reduced or switched off in the same manner asdescribed in FIG. 1. The technology described above is thus possible touse on two-dimensional antenna arrays

FIG. 11 shows a frontal radiation diagram of an antenna array 1 systemaccording to FIG. 10. The displacement of the grating lobes 7 towardsthe centre of the main lobes 6 has been depicted as a number of gratinglobes 7 on an Z-axis on opposite sides of the main lobes 6 and a numberof grating lobes 7 on an Y-axis on opposite sides of the main lobes 6.The displacement of the grating lobes 7 along the Z-axis corresponds tothe removal of antenna elements 2 along the Z-axis in FIG. 10. Thedisplacement of the grating lobes 7 along the Y-axis corresponds to theremoval of antenna elements 2 along the Y-axis in FIG. 10.

The technology may be used for a two-dimensional array as depicted inFIG. 10. FIG. 11 shows that the reduction of the two-dimensional antennaarray 1 gives diminished ranges for the grating lobes 7 and a diminishedwidth of the main lobe, as for the one-dimensional antenna array 1 asdepicted in FIG. 1. The grating lobes 7 may thus be rejected in atwo-dimensional antenna array 1 system according to the above method.

FIG. 12 shows a block diagram over the method according to one exampleembodiment. The blocks in FIG. 12 depict a number of means suitable forperforming the method.

FIG. 12 shows an antenna array 1 system 23 comprising means forenhancing the measuring accuracy in an antenna array 1 comprising anumber of antenna elements 2. The antenna array 1 system 23 comprises;

-   -   means 13 for receiving analog signals with the antenna array 1        elements, and;    -   means 14 for producing values for a radiation diagram from the        signals.

The antenna array 1 system 23 is characterized in that the antenna array1 comprises;

a)—means 13 for receiving analog signals on all antenna elements 2 at afirst time t₁;

-   -   means 14 for producing first values for a first radiation        diagram from the values in the signals from the first time t₁,        and;    -   means 15 for finding the maximum point 8 for the first values,

b)—means 16 for switching off or reducing the signal from oneinteradjacent antenna element 2 at a second time t₂;

-   -   means 13 for receiving analog signals on all antenna elements 2        except from the one switched off or reduced antenna element,        and;    -   means 14 for producing second values for a second radiation        diagram from the values in the signals from the second time t₂;

c)—means 17 for using the first values to calculate a first range 9referring to the second radiation diagram, outside which first range 9grating lobes 7 will appear in the second radiation diagram;

-   -   means 18 for rejecting all values outside the first range 9,        and;    -   means 15 for finding the maximum point 8 for the second values.

When using a full array there will be no grating lobes which is why theantenna array 1 system 23 will not calculate a range nor will the systemreject any grating lobes at. Therefore, in FIG. 12, the means 17 forusing values to calculate a range and means 18 for rejecting all valuesoutside such a range, will not be used for the first cycle.

The system comprises means 19 for repeating act b) and act c) such thatthe antenna configuration dynamically is altered such that interadjacentantenna elements 2 are switched off or reduced until only the outermostantenna elements 2 remain.

The means 15 for finding the maximum point 8 for the values comprisesmeans 20 for calculating at what angle θ_(max) the maximum point 8 forthe main lobe 6 appears in a radiation diagram.

The system comprises means 21 for converting the analog signals todigital signals by sampling.

The system comprises means 22 for producing a radiation diagram from thevalues.

The antenna elements 2 have a relative distance 3 such that no gratinglobes 7 will occur when using all elements in a full array.

The above mentioned means may be any suitable devices for handlingsignals and for performing mathematical tasks, for example a computer.

The embodiments above have been described as removing a number ofantenna elements 2, but by removing it is to be understood that theantenna elements 2 are dampened or reduced rather than removed.

Furthermore the invention can be used for the same frequency between thedifferent times or for different frequencies.

The invention is not limited to the embodiments above, but may beamended within the scope of the claims.

1. A method for enhancing the measuring accuracy in an antenna arraycomprising a number of antenna elements, the method comprising;a)—receiving analog signals on all antenna elements of the antenna arrayat a first time t₁; producing first values for a first radiation diagramfrom values in the signals from the first time t₁, and; finding amaximum point for the first values, b)—switching off or reducing thesignal from one interadjacent antenna element at a second time (t₂);receiving analog signals on all antenna elements except from the oneswitched off or reduced antenna element, and; producing second valuesfor a second radiation diagram from values in the signals from thesecond time (t₂); c)—using the first values to calculate a first rangereferring to the second radiation diagram, outside which the first rangegrating lobes will appear in the second radiation diagram; rejecting allvalues outside the first range, and; finding a maximum point for thesecond values.
 2. The method according to claim 1, further comprisingrepeating act b) and act c) whereby an antenna configuration isdynamically altered such that interadjacent antenna elements areswitched off or reduced until only the outermost antenna elementsremain.
 3. The method according to claim 1, wherein the act of findingthe maximum point for the values comprises calculating at which angle(θ_(max)) the maximum point for the main lobe appears in a radiationdiagram.
 4. The method according to claim 1, further comprisingconverting the analog signals to digital signals by sampling.
 5. Themethod according to claim 1, further comprising producing a radiationdiagram from the values.
 6. The method according to claim 1, wherein theantenna elements have a relative distance such that no grating lobesoccur when using all elements in a full array.
 7. An antenna arraysystem comprising: an antenna array comprising a number of antennaelements; means for receiving analog signals with the antenna arrayelements, and; means for producing values for a radiation diagram fromthe signals, a) means for receiving analog signals on all antennaelements of the antenna array at a first time (t₁); means for producingfirst values for a first radiation diagram from values in the signalsfrom the first time (t₁), and; means for finding a maximum point for thefirst values, b) means for switching off or reducing the signal from oneinteradjacent antenna element at a second time (t₂); means for receivinganalog signals on all antenna elements except from the one switched offor reduced antenna element, and; means for producing second values for asecond radiation diagram from values in the signals from the second time(t₂); c) means for using the first values to calculate a first rangereferring to the second radiation diagram, outside which first rangegrating lobes appear in the second radiation diagram; means forrejecting all values outside the first range, and; means for finding amaximum point for the second values.
 8. An antenna array systemaccording to claim 7, further comprising means for repeating act b) andact c) whereby that an antenna configuration is dynamically altered suchthat interadjacent antenna elements are switched off or reduced untilonly the outermost antenna elements remain.
 9. An antenna array systemaccording to claim 7, further comprising means for finding the maximumpoint for the values comprises means for calculating at what angle(θ_(max)) the maximum point for the main lobe appears in a radiationdiagram.
 10. An antenna array system according to claim 7, furthercomprising means for converting the analog signals to digital signals bysampling.
 11. An antenna array system according to claim 7, furthercomprising means for producing a radiation diagram from the values. 12.An antenna array system according to claim 7, wherein the antennaelements have a relative distance such that no grating lobes (7) willoccur when using all elements in a full array.
 13. A computer programproduct comprising instructions stored on a storage medium which, whenexecuted, perform the acts of: receiving analog signals on all antennaelements of an antenna array at a first time t₁; producing first valuesfor a first radiation diagram from values in the signals from the firsttime t₁; finding a maximum point for the first values, switching off orreducing the signal from one interadjacent antenna element at a secondtime (t₂); receiving analog signals on all antenna elements except fromthe one switched off or reduced antenna element; producing second valuesfor a second radiation diagram from values in the signals from thesecond time (t₂); using the first values to calculate a first rangereferring to the second radiation diagram, outside which the first rangegrating lobes will appear in the second radiation diagram; rejecting allvalues outside the first range, and; finding a maximum point for thesecond values.